Section 173-360A-0625 - Weekly manual tank gauging.Owners and operators using weekly manual tank gauging must comply with the requirements of this section.
(1)Standards.(a) Weekly manual tank gauging must be performed in a manner that is able to detect any leaks greater than the applicable test standards in Table 0625-1 with a probability of detection of at least 0.95 and a probability of false alarm of no more than 0.05.(b) Weekly manual tank gauging must be performed using a gauge stick or an automatic tank gauge system that is able to measure the following:(i) Tank liquid levels over the full range of the tank's height to the nearest one-eighth of an inch; and(ii) Water levels in the bottom of the tank to the nearest one-eighth of an inch.(2)Performance. Weekly manual tank gauging must be performed in accordance with the requirements of this subsection. Automatic tank gauge systems must be operated in accordance with the manufacturer's instructions. (a) Each week, the following inventory volume measurements and calculations must be performed: (i) Select a period that is at least as long as the applicable minimum test duration specified in Table 0625-1. During this period, no liquid may be added or removed from the tank;(ii) Take two consecutive tank liquid level measurements to the nearest one-eighth of an inch at the beginning and at the end of the selected period;(iii) Calculate the average of the two beginning measurements and the average of the two ending measurements and convert the averages from inches to gallons;(iv) Calculate the change in volume by subtracting the average ending measurement from the average beginning measurement; and(v) Compare the change in volume to the applicable weekly standard in Table 0625-1.(b) Each month, calculate the monthly change in volume by adding together the four weekly changes in volume and compare to the applicable monthly standard in Table 0625-1.(c) At least once each month, the level of any water in the tank must be measured to the nearest one-eighth of an inch.(3)Suspected release. A release is suspected based on weekly manual tank gauging if: (a) Any weekly change in volume is greater than the applicable weekly standard in Table 0625-1;(b) Any monthly change in volume is greater than the applicable monthly standard in Table 0625-1; or(c) The presence of water in the tank is unexplained. Table 0625-1: Weekly Manual Tank Gauging Criteria
| Nominal Tank Capacity | Minimum Test Duration | Weekly Standard (one test) | Monthly Standard (four test average) |
| 550 gallons or less | 36 hours | 10 gallons | 5 gallons |
| 551 - 1,000 gallons(when tank diameter is 64 inches) | 44 hours | 9 gallons | 4 gallons |
| 551 - 1,000 gallons(when tank diameter is 48 inches) | 58 hours | 12 gallons | 6 gallons |
| 551 - 1,000 gallons (when tank diameter other than 64 or 48 inches) | 36 hours | 13 gallons | 7 gallons |
| 1,001 - 2,000 gallons | 36 hours | 26 gallons | 13 gallons |
Wash. Admin. Code § 173-360A-0625
Statutory Authority: Chapter 90.76 RCW. WSR 18-15-083 (Order 16-02), § 173-360A-0625, filed 7/18/18, effective 10/1/18.